You just need to run one of the leads to a diode which feeds a capacitor. The other lead of the capacitor connects to the second Piezo element lead. You won't get huge power out of it - the element can only flex a little bit without cracking and each flex produces just a little charge which ends up on the capacitor. Since the element only generates a small amount of charge on each flex, to get a lot of charge, requires the element to be rapidly flexed. You can see a demonstration of me vibrating an element like this with an old oscillating razor to produce enough current to illuminate a couple of LEDs. They will generate pretty high voltages if there is no load.